I was preparing for exam and I came across this question, sorry it may not be suitable to ask in this forum but I need a little help.

UCLES 03 O level Bio. 5093/2 Dec. Q.5Suggest an explanation for the fact that the chemicals produced by endocrine glands are usually in the form of small molecules.

In fact, I don't even know what does it mean by small molecules. Insulin is a hormone, but it is also a protein. If it's a protein, then is it a small molecule? I can't interpret this question very well, can somebody help me please?

I am no expert on the Endocrine system, this seems like a oddly worded question. The first thing I would think of though is relative molecule size, for example Insulin is gigantic when compared to the size of considerably small molecules such as water. Is Insulin a relatively small protein? I don't know the answer to that, but I would assume due to the question that the molecules produced by the Endocrine system are small.

Some possibilities are that small Hydrophilic that are produced molecules also help solubility through plasma membranes, which is sometimes a requirement for Endocrine molecules and assisting with diffusion through blood vessels which is always a requirement. - Due to it being a O-level question it is probably to do with diffusion or so (I would assume).

Depending if it's a Neuroendocrine molecule it will need to reach a minimum threshold or exceed that threshold of receptor-molecule complexes in order to produce a response, small physical size makes it more probable for them to bind with ease.

One other possibility is that Endocrine molecules tend to be cell-signalling molecules (as far as I know), therefore there's no real need for the molecules to be excessively big as long as they can attach to the specific active site.

I hope this all helps, as I said, I am no expert in this field these are just my first thoughts on the situation. Probably best to check out the Endocrine system for a biological book, Campbell Biology is a good overview of Biology.